fix(session-index): bound FTS rebuild memory

[AstroQore]

Summary

• rebuild the full session index in bounded 4 MiB / 128-session FTS batches
• release SQLite and malloc caches after every committed batch
• keep search scopes, excerpts, snippets, and per-session indexing policy unchanged
• force a schema-v5 derived-index rebuild so existing pathological segment layouts are replaced

Verification

• targeted session-index suites: 45 tests passed
• full package suite: 567 tests across 81 suites passed
• git diff --check

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 4a9739b003

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Keep SQLite temporary files in the caller-provided directory

When FTS indexing or a query needs temporary storage, forcing temp_store=FILE makes SQLite create temporary files in its process-wide OS-selected temp directory rather than beside the caller-provided database URL. This breaks the library's filesystem boundary and can write session-derived data somewhere the caller never authorized; use memory-backed temporary storage or another mechanism that confines spill files to the supplied directory.

AGENTS.md reference: AGENTS.md:L88-L89

Useful? React with 👍 / 👎.